Antenna elements and systems, and other radio and microwave devices, are conventionally constructed from conductive radiating elements, transmission lines and ground planes, and non-conductive spacers, mechanical supports, and other components. Their design typically requires the selection of appropriate materials. Size, weight, electrical properties and environmental resistance are primary parameters of interest.
For example, the current trend in mobile antenna designs, such as those required by aircraft, ships and other vehicles, result in a need for low profile, directional antenna configurations which can conveniently be made to conform to the shape of a mobile unit, such as an airplane wing, while providing excellent beam steering and electromagnetic properties. Moreover, safety and fuel economy have become important factors in vehicle mounted antenna design. Projections from mobile antennas mounted on such vehicles are not only hazardous, but also cause drag and instability to the vehicle and vibration while the vehicle is in motion.
However, radiating elements must typically be positioned at least one quarter wavelength away and parallel to a ground plane (such as the metallic skin of an aircraft) to prevent unwanted cancellation between the radiated signal from the radiating elements and the reflected signals from the ground plane. When the plane of the elements are brought closer to the ground plane, the reflected waves from the surface interferes with the directed waves, producing a loss in signal strength and in radiation efficiency. Placing a dielectric substrate having a high dielectric constant between the ground plane and the plane of the elements has been used to minimize such losses. When a high dielectric material is placed between the ground plane and the radiating element, the incident radiation is slowed down by the index of refraction (H) of the material; however, increasing the dielectric constant (relative permittivity--.epsilon..sub.r) to 10 or more without a similar increase in relative permeability (.mu..sub.r) results in a severe impedance mismatch and thus is not technically desirable for many broadband applications.
To achieve sufficient bandwidth, conventional meander-line polarizers require multiple layers of material spaced at least one quarter wavelength apart, and thus tend to be a wavelength in length or longer. When such devices are applied to apertures which are less than approximately one wavelength in size or when they are forced into the flares of small horns, a serious deterioration in performance results. Conventional radio and microwave frequency polarizers are also subject to losses caused by high loss tangents and severe impedance mismatching at the entrance and exit ports. Prior art radio frequency and microwave lenses and other electromagnetic devices operating in the radio and microwave frequency ranges suffer from similar drawbacks.
It is known to reduce the size of a conventional loop or whip antenna by embedding it in a ferrite loading material. Although it utilizes materials which are quite lossy, such a loaded design more than compensates for the mismatch losses that would otherwise result between the maximum practical antenna size for a portable AM radio (tens of centimeters) or other handheld device designed to receive a signal in the kilohertz range, and the optimal antenna size that would be required as those frequencies (tens of meters) in the absence of any loading material.
It has also been proposed to use a commercially available surface wave absorber material having a relatively high refractive index to microwave radiation as a low propagation velocity material between various planar radiating elements of a broadband antenna and their respective ground planes; however, the heretofore known such materials had a relatively high loss tangent (on the order of 0.3) and the resultant efficiency is an order of magnitude less than acceptable for most commercial applications.
U.S. Pat. No. 3,540,047 (Walser) discloses radiation absorbing layers forming a three dimensional array of thin ferromagnetic elements with all the elements having a common uniaxial anisotropy axis, which is usable at microwave frequencies (200 mHz to 2 gHz). Although the patent hints at the possibility of other uses requiring a "reduced" magnetic loss tangent, the disclosed examples are intended only to absorb incident radiation and do not appear to have either matched values of relative permittivity and relative permeability, or low magnetic loss tangents, at the microwave frequencies of interest. U.S. Pat. No. 5,047,296 (Miltenberger) discloses another anisotropic radiation absorption material formed from layers of individual blocks of amorphous magnetic films, with the different layers having crossed magnetic axes. That material also does not appear to have either matched values of relative permittivity and relative permeability, or low magnetic loss tangents. However, at least from the latter patent, it is apparent that the real and imaginary permeability components of the array are primarily dictated by the corresponding properties of the bulk material from which the individual elements are formed, and thus it should be possible to manufacture similar materials with other electrical properties.